As a student at Occidental College in Los Angeles from 1979 to 1981, "There were days where folks couldn't go outside … because of all the pollution in the air."

— President Barack Obama, Tuesday in a speech on climate change

The ruling

In the United States, Los Angeles has been ground zero for smog for decades, due to its heavy automobile usage, its industrial base and the presence of mountains and valleys, which prevent polluted air from floating elsewhere. Los Angeles smog emerged in the 1940s and worsened from the 1950s to the 1970s. It got so bad in October 1954 that the city virtually shut down for the entire month.

In rising order of seriousness, smog alerts for southern California start with health advisories and move upward to stage 1, stage 2 and stage 3. The last stage 3 smog alert came in 1974. The last stage 2 alert came in 1988, and there has been only one stage 1 alert since 1998 (it came in 2003).

So what would Obama have experienced when he was a student at Occidental, the college he attended before transferring to Columbia University for his junior and senior year?

According to South Coast Air Quality Management District data, a health advisory — a comparatively low-level alert — was issued somewhere in the agency's region of jurisdiction 169 times in 1979, 152 times in 1980 and 159 times in 1981. The stage 1 threshold was met 120, 101, and 99 times during those three years.

Even allowing that Occidental itself may have experienced just a fraction of these episodes, the university still would have faced a significant number.

So what did these advisories mean in a practical sense? For a health advisory, the lowest of these levels, all children are supposed to "discontinue prolonged, vigorous outdoor exercise lasting longer than one hour," and "susceptible persons, such as those with heart or lung disease" should "avoid outdoor activity," including "calisthenics, basketball, running, soccer, football, tennis, swimming laps, and water polo."

For the more severe stage 1 episodes, all children are supposed to "discontinue all vigorous outdoor activities regardless of duration," including "physical education classes, sports practices, and athletic competitions."

Finally, for stage 2 or stage 3, all children are supposed to "discontinue all outdoor activities."

Obama said that "there were days where folks couldn't go outside," but nothing in the rules addressed what healthy adults could do, including healthy college-age people. Rather, the rules addressed adults with compromised health as well as children.

Still, the reality of smog alerts was not pretty, and by all accounts, staying inside, if feasible, was certainly preferable.